- Metabolites like lactate, protons, and potassium ions are produced by the muscles during hard exercise, and are carried throughout the body in your circulating blood. These metabolites may directly inhibit the function of muscles they come in contact with.

- The same metabolites may be detected by sensors that convey the message to your brain, which responds by reducing the signals it sends to muscles throughout the body.

- Mental fatigue from the first bout of exercise makes you less able/willing to focus and exert the same degree of effort as you would if you were mentally fresh.

This is a big topic of research these days, because if we can understand how nonlocal fatigue works, we'll be a lot closer to understanding how plain old local fatigue works too. On that note, there's a new study out from Israel Halperin and his colleagues at Memorial University of Newfoundland (who also produced the previous study I blogged about), published in Applied Physiology, Nutrition, and Metabolism. I regret to say that it doesn't make things any simpler, but it does add to what we know about the topic.

They did two sets of experiments, with a bunch of different protocols. Here's my attempt to give a simple overview:

Condition 1: Fatigue right leg, test left leg

Condition 2: Fatigue right leg, test left arm

Condition 3: Fatigue right arm, test left leg

Condition 4: Fatigue right arm, test left arm

The actual protocol was a little more involved than that, because they also had rest/control groups to compare each condition to, and they used several measures of fatigue (i.e. single max voluntary contraction vs. 12 repeated max contractions; strength vs. EMG; etc.). Also, they specified dominant and non-dominant arms, not left and right. But hopefully you get the point!

Anyway, the basic result was this: Whether you fatigue an arm or a leg, the opposite leg will be tired in a subsequent bout of exercise. Conversely, whether you fatigue an arm or leg, the opposite arm will not be tired in a subsequent bout of exercise. So the arms and legs (or at least the elbow flexors and knee extensors) appear to behave differently with respect to nonlocal fatigue. Why would this be? The short answer is the researchers don't know. They discuss some theories, like the fact that leg muscles have more motor units and more fast-twitch fibers, making them more difficult to fully activate than arm muscles. It may also be that the neural wiring that activates and monitors the legs is different because sensory feedback is so important for muscles involved in walking, running, and balance.

A caveat: this doesn't mean that arms are totally immune to nonlocal fatigue. In fact, the earlier Halperin study I blogged about did find nonlocal fatigue in arms. The key difference, as you'd expect, is that the subjects were a lot more fatigued in the earlier study. They did five sets of knee extensions to exhaustion before testing their arms, whereas the new study only involved a 100-second maximal contraction performed twice before testing for nonlocal effects.

The researchers also mention very briefly the potential training implications of their findings: If arms are less susceptible to nonlocal fatigue than legs, then it might make sense to do leg exercise first if you're combining both in one session. It's a pretty big stretch to translate a study like this into training advice -- but it's food for thought.